What factors have turned ferns into fossil fuels over thousands of years?

The factors that turned ferns into fossil fuels over thousand of years are pressure and heat. Ferns live in swamp forests, when they died and decayed they formed layers at the bottom of the swamp. Soil and water will built up and the heat and pressure would cause the decayed ferns to change over thousands of years, continuing to decay in the absence of oxygen will turn the fern into fossil fuels.

It is just pressure and heat my mans